The present invention relates to stator winding splice joints and, more particularly, to welded stator winding splice joints utilizing gas tungsten arc welding to provide high performance and reliability.
A rotary electric machine such as a turbine generator generally comprises a stator having an iron core and a coil winding, and a rotor rotatably supported within the stator and having a core and coil winding. The annular stator core has a plurality of axially extended core teeth to form coil slots therebetween at regular intervals circumferentially of the inner surface of the stator core.
The function of the stator winding is to provide a well defined path for electric current flow to and from an external system at a prescribed terminal voltage level and temperature level. The stator winding coils are made up of bundles of strands of insulated copper wires that are disposed in the slots of the stator core. Referring to FIG. 1, at the ends of each coil, the copper wires are brazed together to form bar leads 10, 12. A pair of coils is connected by sandwiching the stator bar leads 10, 12 with two connection straps 14, 16 and brazing them together, as schematically shown at 18, to provide an electric current path. There are several dozen such conventional stator winding splice joints contained in a rotary electric machine. The failure of one splice joint can result in the failure of the entire stator winding. Thus, each splice joint plays an important role in ensuring the normal operation of the rotary electric machine. The reliability of each splice joint has a direct and strong impact on the reliability of the whole machine.
As noted above, brazing is conventionally used to join the stator coils. During brazing, the base metal is not melted. Thus, the filler metal is chosen to melt at a lower temperature than the base metal. Although brazing is widely used, brazing has numerous disadvantages. For example, because good brazing depends on the capillary flow of the filler metal, surface cleanliness in brazing is much more critical than other joining processes, such as welding. Any contamination on the brazing surfaces may cause the ultimate failure of the splice joint. Further, the filler material in many brazed joints is considerably weaker than the joint base material. In addition, brazing produces waste disposal and hazards that are harmful to the environment. Furthermore, distortion can be experienced in brazing and the evaluation of strength of brazed joints is more complex than that of welded joints. In summary, brazing usually has a relatively low reliability. In spite of these shortcomings, the conventional design of the stator winding splice joints employs brazing as the joint method.
Gas tungsten arc welding (GTAW) is a high-precision, high-quality, high reliability, low cost and simple joining process. GTAW produces the coalescence of metals by heating them with an arc between a non-consumable tungsten-electrode and the base metal. During the welding process, an inert gas such as helium sustains the arc and protects the molten metal from atmospheric contamination and oxidation. The advantages of GTAW include the fact that it produces superior quality welds that are generally free of defects; it is free of the spatter that occurs in brazing processes; it can be used with or without a filler metal; it can be used with a wide range of power supplies; it allows precise control of welding penetration depths; it can produce inexpensive autogenous welds at high speeds; and it can be used to weld almost all metals.
As noted above, conventional splice joints employ brazing as the joining method. Because of the strict cleaning requirements for the brazing surfaces, the brazing process is rather time consuming and costly. It therefore would be desirable to modify the stator winding splice joining process to increase the reliability of stator winding splice joints in rotary electric machines. It would also be desirable to simplify the manufacturing processes and reduce assembly cycle time. It would also be desirable to reduce labor material and reworking costs and to provide a technique for joining the winding that reduces the production of waste and hazards, in particular compared to the conventional brazing process. The invention addresses the aforementioned problems of the conventional brazing process and in particular is embodied in the use of a GTAW process to weld stator winding splice joints to increase manufacturing efficiency and maintain product quality stability.